This invention relates to an apparatus for development of electrostatic latent images created, for example, in a copier or a printer.
In electrostatic recording or reproducing processes such as electrophotography, electrostatic recording and photocopying and ionography utilized in the current copiers, printers, etc., the fundamentals are creating an electrostatic latent image on a carrier surface and developing the latent image into a visible image by deposition of a colored substance. For creating an electrostatic latent image there are various methods such as, for example, selective discharge of a uniformly charged photoconductive carrier by exposure to light and impartment of a preformed electrostatic charge to a dielectric carrier with a multi-pin electrode or an ion discharge gate. Further, fixation of the developed image is performed in various ways depending on the relation between the ultimate recording medium and the carrier on which the latent image is formed. In some cases the ultimate recording medium itself is used as the carrier, and in some other cases the developed image is transferred onto the ultimate recording medium before fixation. However, irrespective of such variations it is indispensable and fundamentally invariable to develop an electrostatic latent image into a visible image by application of a colored developer.
The currently prevailing method of developing electrostatic latent images is the dry development method using a powder toner. A representative of the dry development method is the magnetic brush development method in which the toner is conveyed to the development zone by magnetic force. However, a disadvantage of the dry development method is that the apparatus is liable to be soiled with a cloud of the toner powder. Besides, the visible image formed of the powder toner has to be fixed to the recording medium by application of heat and/or pressure.
It is also possible to develop electrostatic latent images by a wet process using a liquid developer. In a popular wet development method the liquid developer is a dispersion of colored particles in an organic liquid of high resistivity. A recording medium with latent images thereon is immersed in the liquid developer, and development is effected by electrophoretic deposition of the colored particles on the latent images. To complete the development process the organic liquid must be evaporated from the recording medium. Therefore, it is necessary to take some measures for preventing concentration of the vapor of the organic liquid in or around the development apparatus. As another problem with this wet development method, it is often that some of the colored particles in the liquid developer adhere to the uncharged areas of the recording medium surface to result in smudging or discoloration of the background.
To solve such problems, U.S. Pat. No. 4,202,620 proposes a liquid development method which is characterized in that a film of a liquid developer on a solid surface is brought close to a carrier surface on which an electrostatic latent image is formed. Although the liquid developer film does not come into contact with the carrier surface, the electrostatic field created by the charge on the carrier surface raises a local bulge in the liquid film in the region opposed to the latent image, and a slender projection grows up from the bulge as the gap between the liquid film and the carrier surface narrows, until the peak of the projection touches the latent image. In this manner the liquid developer adheres to the carrier surface only in areas where latent images exist. Therefore, development is accomplished without smudging the background areas and without adhesion of excessive liquid medium. Besides, in this method it is possible to use an aqueous dispersion as the liquid developer to thereby avoid generation of a vapor of an organic solvent at the subsequent drying stage.
However, a shortcoming of this method resides in that the bulges in the liquid developer film are not always created at accurately desired locations. That is, each bulge is created at a location where the liquid film makes a tiny protuberance due to minute swaying of the free surface of the film with resultant concentration of the electrostatic field between that protuberance and the latent image on the carrier surface. Therefore, in a relatively large area opposed to a latent image to be developed into a uniformly solid black region the bulges in the liquid film are not uniformly distributed over that area, so that the developed image contains uncolored regions and does not become uniformly solid black. As another problem, once a bulge is created in the liquid film and a slender projection begins to grow up from the bulge, further concentration of the electrostatic field occurs in that location. Therefore, in an area opposed to a plurality of tiny and closely formed latent images it is seldom that a projection of the liquid film grows up for each of such latent images. Instead, it is likely that a first created projection alone continues to grow up and comes into contact with more than one of the closely formed latent images. For this reason it is difficult to clearly reproduce fine images without blurring and without destroying necessary spacings.